A user-friendly calculator for determining oxygen isotope composition of body water in cows based on the MK model and examples of its applications

Abstract

Oxygen isotope composition (δ18O) of body water or protein shows potential for tracing production systems in animal products like milk. However, parameters influencing this value are difficult to investigate by controlled experiments due to fluctuating ambient conditions and due to the slow body water turnover that requires large amounts of isotopically constant diet (e.g. drinking water and feed moisture) to achieve equilibrium conditions. A mechanistic model (MK model) was developed as a promising assistant to predict environmental, physiological and animal management related influences on δ18O of body water. A user-friendly spreadsheet calculator (MK calculator) allows running this model. Measured data of milk and hair were used for validation and showed a root mean squared error of 1.3‰ (n = 796) and a Nash-Sutcliffe modeling efficiency of 95%. Examples of application are divers like (1) evaluating the interaction of ambient conditions with feeding strategies; (2) looking into influences of fraction of grass silage and its δ18O on δ18O of body water; (3) investigating the influence of moisture contents in grass, grass silage and hay; (4) predicting the influence of drinking water on δ18O of body water depending on feeding strategy; (5) assessing the half-life of body water depending on physiological state and ambient conditions. The MK calculator may be applied for a wide range of questions related to δ18O of body water like influences of animal behavior, of diseases, or of transhumance. The MK calculator is also easy to adapt to other animals than cows.